Nelson Rowell - Academia.edu (original) (raw)

Papers by Nelson Rowell

Angewandte Chemie International Edition

Colloidal semiconductor II–VI metal chalcogenide (ME) magic‐size clusters (MSCs) exhibit either a... more Colloidal semiconductor II–VI metal chalcogenide (ME) magic‐size clusters (MSCs) exhibit either an optical absorption singlet or doublet. In the latter case, a sharp photoluminescence (PL) signal is observed. Whether the PL‐inactive MSCs transform to the PL‐active ones is unknown. We show that PL‐inactive CdS MSC‐322 transforms to PL‐active CdS MSC‐328 and MSC‐373 in the presence of acetic acid (HOAc). MSC‐322 displays a sharp absorption at ≈322 nm, whereas MSC‐328 and MSC‐373 both have broad absorptions respectively around 328 and 373 nm. In a reaction of cadmium myristate and S powder in 1‐octadecene, MSC‐322 develops; with HOAc, MSC‐328 and MSC‐373 are present. We propose that the MSCs evolve from their relatively transparent precursor compounds (PCs). The PC‐322 to PC‐328 quasi‐isomerization involves monomer substitution, while monomer addition occurs for the PC‐328 to PC‐373 transformation. Our findings suggest that S dominates the precursor self‐assembly quantitatively, and li...

Topics in Applied Physics, 2021

This Chapter discusses the phenomena associated with germanium (Ge) nanocrystals emitting near in... more This Chapter discusses the phenomena associated with germanium (Ge) nanocrystals emitting near infrared radiation under optical excitation. We describe how the emission properties are influenced by various effects, including those of strain and particle confinement, as well as excitation mechanisms. Two example systems are discussed, namely one of isolated Ge quantum dots and another of Ge nanocrystals coherently imbedded in SiGe alloy layers, where both systems were grown by molecular beam epitaxy (MBE) on Si substrates. For the Ge dot ensembles, we show how particle size information can be derived from the emission spectrum. For the Ge nanocrystals, the emission spectra are analyzed for the effects of strain and particle confinement over a wide range of Ge fractions in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape, which were a 1.4 nm thickness in the MBE growth direction and a 7 nm lateral dimension. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures.

LEOS '90. Conference Proceedings IEEE Lasers and Electro-Optics Society 1990 Annual Meeting

arXiv: Instrumentation and Detectors, 2005

We have studied the accuracy of the atomic microwave power standard. The atoms are cooled and kep... more We have studied the accuracy of the atomic microwave power standard. The atoms are cooled and kept in a Magneto-Optical Trap (MOT), then dropped through a terminated transmission line (a rectangular, R-70 type, waveguide). The measurement of the internal atomic state allows an accurate determination of the transmitted microwave power.

Accounts of Chemical Research, 2021

ConspectusA knowledge of colloidal semiconductor magic-size clusters (MSCs) is essential for unde... more ConspectusA knowledge of colloidal semiconductor magic-size clusters (MSCs) is essential for understanding how fundamental properties evolve during transformations from individual molecules to semiconductor quantum dots (QDs). Compared to QDs, MSCs display much narrower optical absorption bands; the higher cluster stability gives rise to a narrower size distribution. During the production of binary QDs such as II-VI metal (M) chalcogenide (E) ones, binary ME MSCs observed were interpreted as side products and/or the nuclei of QDs. Prior to the current development of our two-step approach followed by our two-pathway model, it had been extremely challenging to synthesize MSCs as a unique product without the nucleation and growth of QDs. With the two-step approach, we have demonstrated that MSCs can be readily engineered as a sole product at room temperature from a prenucleation stage sample, also called an induction period (IP) sample. It is important that we were able to discover that the evolution of the MSCs follows first-order reaction kinetics behavior. Accordingly, we proposed that a new type of compound, termed as "precursor compounds" (PCs) of MSCs, was produced in an IP sample. Such PCs are optically transparent at the absorption peak positions of their MSC counterparts as well as to longer wavelengths. It is thought that quasi isomerization of a single PC results in the development of one MSC.In this Account, we provide an overview of our latest advances regarding the transformations among binary CdE MSCs as well as from binary CdTe to ternary CdTeSe MSCs. Optical absorption spectroscopy has been employed to study these transformations, all of which display well-defined isosbestic points. We have proposed that these MSC to MSC transformations occur via their corresponding PCs, also called immediate PCs. It is reasonable that the as-synthesized PC (in an IP sample) and the immediate PC (in an incubated and/or diluted sample) probably have different configurations. A transformation between two PCs may involve an intermolecular reaction, with either first-order reaction kinetics or a more complicated time profile. A transformation between one immediate PC and its counterpart MSC may contain an intramolecular reaction. The present Account, which addresses the PC-enabled MSC transformations with isosbestic points probed by optical absorption spectroscopy, calls for more experimental and theoretical attention to understand these magic species and their transformation processes more precisely.

Inorganic Chemistry, 2021

We report the first observation of the reversible transformations that occur among three types of... more We report the first observation of the reversible transformations that occur among three types of CdTe magic-size clusters (MSCs) in dispersion at room temperature and discuss our understanding of the transformation pathway. The reversible transformations were achieved with CdTe prenucleation stage samples, which were prepared with reactions of cadmium oleate [Cd(OA)2] and tri-n-octylphosphine telluride in 1-octadecene and were then dispersed in mixtures of toluene and a primary amine at room temperature. Three types of OA-passivated CdTe MSCs evolved, exhibiting sharp optical absorption singlets peaking at 371, 417, and 448 nm. The MSCs and their immediate precursor compounds (PCs; with no sharp optical absorption) are labeled by the MSC absorption peak wavelengths. The transformation between MSC-371 and MSC-417 has a distinct isosbestic point at ∼385 nm and that between MSC-417 and MSC-448 at ∼430 nm. Our findings suggest that these PC-enabled reversible transformations occur through a process of quasi-isomerization, transforming between PCs and their counterpart MSCs, combined with substitution reactions that cause transformation between the two involved PCs.

There is a continuing, urgent need for an ophthalmic (eye) drop for the clinical therapy of age-r... more There is a continuing, urgent need for an ophthalmic (eye) drop for the clinical therapy of age-related macular degeneration (AMD), a leading cause of blindness. Here, we report the first formulation of an eye drop that is effective via autophagy for AMD treatment. This eye drop is based on a single natural product derivative (ACD), which is an amphiphilic molecule containing a 6-aminohexanoate group (H 2 N(CH 2) 5 COO−). We demonstrate that this eye drop reverses the abnormal angiogenesis induced in a primate model of AMD that has the pathological characteristics close to that of human AMD. The ACD molecule was self-assembled in an aqueous environment leading to nanoparticles (NPs) about 9.0 nm in diameter. These NPs were encapsulated in calcium alginate hydrogel. The resulting eye drop effectively slowed the release of ACD and displayed extended release periods in both simulated blood (pH 7.4) and inflammatory (pH 5.2) environments. We show that the eye drop penetrated both the corneal and blood−eye barriers and reached the fundus. With low cellular toxicity, the drop targeted S1,25D 3-membrane-associated rapid response steroid-binding protein (1,25D 3-MARRS) promoting autophagy in a dose-dependent manner. In addition, the drop inhibited cell migration and tubular formation. On the other hand, when protein 1,25D 3-MARRS was knocked down, the eye drop did not exhibit such inhibition functionalities. Our study indicates that the 6-aminohexanoate group on self-assembled NPs encapsulated in hydrogel leads to the positive in vivo outcomes. The present formulation offers a promising approach for clinical treatment of human AMD.

ECS Meeting Abstracts, 2005

not Available.

The Journal of Physical Chemistry Letters, 2019

Reports on photoluminescent colloidal semiconductor two-dimensional (2D) helical nanostructures w... more Reports on photoluminescent colloidal semiconductor two-dimensional (2D) helical nanostructures with one-dimensional quantum confinement are relatively rare. Here, we discuss the formation of such photoluminescent nanostructures for CdSe. We show that when as-synthesized unpurified zero-dimensional (0D) CdSe magic-size clusters (MSCs) (passivated by carboxylate ligands with three-dimensional quantum confinement) are dispersed in a solvent (such as toluene or chloroform) containing hexadecylamine and then subjected to sonication, helical nanostructures are obtained, as observed by transmission electron microscopy. We demonstrate that the formation involves the self-assembly of 0D MSCs into 2D nanoplatelets, which act as intermediates. The CdSe MSCs and their self-assembled 2D nanostructures display almost identical static optical properties, namely, a sharp absorption doublet with peaks at 433 and 460 nm and a narrow emission peak at 465 nm; this is a subject for further study. This study introduces new methods for fabricating photoluminescent helical nanostructures via the self-assembly of photoluminescent MSCs.

Nature Communications, 2019

Alloy semiconductor magic-size clusters (MSCs) have received scant attention and little is known ... more Alloy semiconductor magic-size clusters (MSCs) have received scant attention and little is known about their formation pathway. Here, we report the synthesis of alloy CdTeSe MSC-399 (exhibiting sharp absorption peaking at 399 nm) at room temperature, together with an explanation of its formation pathway. The evolution of MSC-399 at room temperature is detected when two prenucleation-stage samples of binary CdTe and CdSe are mixed, which are transparent in optical absorption. For a reaction consisting of Cd, Te, and Se precursors, no MSC-399 is observed. Synchrotron-based in-situ small angle X-ray scattering (SAXS) suggests that the sizes of the two samples and their mixture are similar. We argue that substitution reactions take place after the two binary samples are mixed, which result in the formation of MSC-399 from its precursor compound (PC-399). The present study provides a room-temperature avenue to engineering alloy MSCs and an in-depth understanding of their probable formati...

Nano Research, 2019

We report, for the first time, the synthesis of CdS magic-size clusters (MSCs) which exhibit a si... more We report, for the first time, the synthesis of CdS magic-size clusters (MSCs) which exhibit a single sharp absorption peaking at ~ 361 nm, along with sharp band edge photoemission at ~ 377 nm and broad trap emission peaking at ~ 490 nm. These MSCs are produced in a singleensemble form without the contamination of conventional quantum dots (QDs) and/or other-bandgap clusters. They are denoted as MSC-361. We present the details of several controlled syntheses done in oleylamine (OLA), using Cd(NO 3) 2 or Cd(OAc) 2 as a Cd source and thioacetamide (TAA) or elementary sulfur (S) as a S source. A high synthetic reproducibility of the reaction of Cd(NO 3) 2 and TAA to single-ensemble MSC-361 is achieved, the product of which is not contaminated by other bandgap clusters and/or QDs. In some cases, the reaction product exhibits an additional absorption peak at ~ 322 nm. We demonstrate that the two peaks, at 361 and 322 nm, do not evolve synchronously. Therefore, the 322 nm peak is not a higher order electronic transition of MSC-361, but due to the presence of another ensemble, namely MSC-322. The present study suggests that there is an outstanding need for the development of a physical model to narrow the knowledge gap regarding the electronic structure in these colloidal semiconductor CdS MSCs.

Advanced Science, 2018

Little is known about the formation pathway of colloidal semiconductor magic‐size clusters (MSCs)... more Little is known about the formation pathway of colloidal semiconductor magic‐size clusters (MSCs). Here, the synthesis of the first single‐ensemble ZnSe MSCs, which exhibit a sharp optical absorption singlet peaking at 299 nm, is reported; their formation is independent of Zn and Se precursors used. It is proposed that the formation of MSCs starts with precursor self‐assembly followed by Zn and Se covalent bond formation to result in immediate precursors (IPs) which can transform into the MSCs. It is demonstrated that the IPs in cyclohexane appear transparent in optical absorption, and become visible as MSCs exhibiting one sharp optical absorption peak when a primary amine is added at room temperature. It is shown that when the preparation of the IP is controlled to be within the induction period, which occurs prior to nucleation and growth of conventional quantum dots (QDs), the resulting MSCs can be produced without the complication of the simultaneous coproduction of conventional...

The journal of physical chemistry letters, Jan 31, 2018

There are two types of colloidal semiconductor nanocrystals (NCs) that exhibit band gap absorptio... more There are two types of colloidal semiconductor nanocrystals (NCs) that exhibit band gap absorption that is relatively sharp compared to conventional quantum dots (QDs). One type displays an absorption doublet, while the other displays an absorption singlet. Here, we report the evolution of the two types of NCs at room temperature from a single CdTe sample extracted during the induction period (IP) prior to nucleation and growth of conventional QDs. The resulting NCs exhibit band gap absorption peaking at ∼371 nm and are magic-size clusters (MSCs), labeled as dMSC-371 and sMSC-371 for the doublet and singlet cases, respectively. We demonstrate that dMSC-371 (with another peak at ∼350 nm) evolves when the sample is incubated. When the sample is dispersed without incubation into a toluene and octylamine mixture, dMSC-371 or sMSC-371 grows depending on the amine amount. We propose that dMSC-371 and sMSC-371 are a pair of polymorphs (with identical CdTe core compositions). The present st...

Nature communications, Jun 27, 2018

Structural isomerism of colloidal semiconductor nanocrystals has been largely unexplored. Here, w... more Structural isomerism of colloidal semiconductor nanocrystals has been largely unexplored. Here, we report one pair of structural isomers identified for colloidal nanocrystals which exhibit thermally-induced reversible transformations behaving like molecular isomerization. The two isomers are CdS magic-size clusters with sharp absorption peaks at 311 and 322 nm. They have identical cluster masses, but slightly different structures. Furthermore, their interconversions follow first-order unimolecular reaction kinetics. We anticipate that such isomeric kinetics are applicable to a variety of small-size functional nanomaterials, and that the methodology developed for our kinetic study will be helpful to investigate and exploit solid-solid transformations in other semiconductor nanocrystals. The findings on structural isomerism should stimulate attention toward advanced design and synthesis of functional nanomaterials enabled by structural transformations.

The journal of physical chemistry letters, Jan 22, 2018

The formation relationship between colloidal magic-size clusters (MSCs) and conventional quantum ... more The formation relationship between colloidal magic-size clusters (MSCs) and conventional quantum dots (QDs) has not been well established. Here, we report our systematic study on their formation pathways, using cadmium sulfide (CdS) as a model system. Two Cd precursors were prepared from CdO with branched 2-methyloctadecanoic acid (CHCH(CH)-COOH) and linear oleic acid (CHCH-COOH), reacting with elemental S powder in 1-octadecene (ODE). We show that the presence of MSC-311 (exhibiting a sharp absorption peaking at 311 nm) is regulated by the growth of conventional QDs. We demonstrate that MSC-311 cannot directly convert into conventional QDs but to its immediate precursor (IP-311), which is transparent in optical absorption (>310 nm). We propose that there are two individual pathways for the formation of MSCs and conventional QDs, linked by an intrinsic pathway from MSCs to IPs to fragments to QDs. The present study introduces new avenues to precisely control their formation.

Solid State Communications, 1994

We have observed visible photoluminescence, at photon energies nearly twice those of the usual ne... more We have observed visible photoluminescence, at photon energies nearly twice those of the usual near-infrared excitonic emissions, from thin Sil.xGe ~ quantum wells at liquid He temperatures. This confirms that a significant biexciton population is present in such samples under excitation conditions normally used for near-infrared photo-luminescence measurements. The intensity of the visible luminescence increases linearly with excitation density, consistent with the biexcitons being localized by fluctuations in alloy content. The biexciton lifetime is observed to vary with the Si~.xGe x quantum well width indicating an enhancement of the overlap of the particle wave functions by the quantum confinement.

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 1996

Si1−xGex epitaxial layers were grown at T=525 °C using a commercially available, ultrahigh vacuum... more Si1−xGex epitaxial layers were grown at T=525 °C using a commercially available, ultrahigh vacuum chemical vapor deposition reactor. Various techniques, including cross-sectional transmission electron microscopy, Auger electron spectroscopy, secondary ion mass spectroscopy, double crystal x-ray diffraction, and photoluminescence (PL) are used to characterize this material. For the first time, phonon resolved PL is used to map out the composition uniformity obtained with this high throughput, production-ready technology. The composition variations along most of the wafer surface (except the flats) do not exceed ±0.15%. A discussion follows on the limitations of this technology, including the critical thickness for misfit strain relaxation, compared to other growth techniques such as molecular beam epitaxy and rapid thermal chemical vapor deposition. The material grown here exhibits characteristics that are very encouraging for the prospect of manufacturing high frequency devices and ...